1. Field of the Invention
This invention relates to an agent for increasing the sugar content and/or decreasing the acid content in plant fruits, which agent has a fluorine-containing .beta.-indolebutyric acid compound as a substantial main component thereof, and to a method for obtaining ripe fruits having a high sugar content and/or a low acid content by causing the fluorine-containing .beta.-indolebutyric acid compound to adhere to unripe fruits, to the environment of plants bearing fruits, or to plants in their entirety, and subsequently allowing the fruits to mature.
2. Description of the Prior Art
The most important requirement for fruits used for luxury consumption in the form of their fresh meats or their extracted juices is abundant sweetness. The sweetness of a fruit is more strongly perceived when the sugar content determined as a physical quantity is large and the acid content similarly determined is small, and the sweetness of a fruit is generally perceived with significant difference when the sugar-acid ratio of the fruit calculated as the ratio of the sugar content to the acid content is higher than that of the compared fruit by one unit.
It is a common practice to treat plants bearing fruits with an agent for decreasing their acid content or an agent for increasing their sugar content for the purpose of imparting increased sweetness to the harvested fruits. Ethyl Kurozeto [sodium 5-chloro-1H-indazol-3-ylacetate or ethyl 5-chloro-1H-indazol-3-ylacetate] has been recognized to be more or less effective as an agent for increasing the sugar content and promoting the coloration of citrus fruits. Among the agents for decreasing the acid content of citrus fruits, lead arsenate is the first to have been registered as an agricultural chemical compound in Japan. It was extensively utilized as an agent for decreasing the acid content of citrus fruits for many years.
Chronic lead arsenate poisoning is, however, known to have caused lung cancer, skin cancer, etc. in large numbers of workers engaged in spraying the agent. In addition, consumers of treated citrus fruits are in danger of suffering oral acute poisoning owing to the lead arsenate remaining on the citrus fruit skin. In 1978, the registration of lead arsenate as an agricultural chemical was canceled in Japan. It is believed that the effect of lead arsenate in decreasing the acid content of citrus fruits arises because this compound disturbs the TCA cycle of not only the fruits but also the whole plant and degrades the level of biosynthesis of citric acid in the plants. The directions attached to lead arsenate packages distributed in the U.S.A., for example, include a warning to the effect that three years' continued use of the compound seriously damages the plants themselves. Even at present, the use of agents which are effective in decreasing acid content is widely recognized as indispensable in the case of citrus fruits of strongly acidic taste. Since no effective substitute for lead arsenate has been developed to date, the use of lead arsenate is still authorized in some countries in spite of the numerous problems of this compound. A strong need is therefore felt for the early development of a substituent which is effective in increasing the sugar content and/or decreasing the acid content in citrus plants and which is safe for the human body.
It is known that 4,4,4-trifluoro-3-(indole-3-)butyric acid, 4,4,4-trifluoro-2-hydroxy-3-(indole-3-)butyric acid, and 4,4,4-trifluoro-3-(indole-3-)butyronitrile have the-ability to promote elongation of plant roots (Collection of monographs for publication at the 1990 general meeting of the Plant Chemical Regulation Society, page 31, published by the Plant Chemical Regulation Society). Besides, fluorine-containing indolebutyric acid compounds which are similarly effective are disclosed in Japanese Patent Publication Hei 5(1993)-279331 filed by some of the present inventors.